The old-old editions of Mutant assumed that there is no interbreeding. The early mutants are described as villagers with 1-3 animal types. Majority dogs and large minorities of fox and cow, for example. Or equally divided between bears and non-mutated humans. The more cosmopolitan society of Pyri, with the first big cities like HIndenburg, was not yet the norm.

I ran the "man on the beach" encounter, where a lab assistant from Eden was discovered on the beach. After a while, one of the slave girls became pregnant. One of the fixers took credit for that and became the hero of the Ark, the only mutant who could give the women babies. This fixer was "the NPC I hate" of one of the PCs, and when he found out the truth about who the father was, he used it to great advantage towards his enemy

In my setting there are a few rules regarding offspring:
1) Humanoids can mate with any other human-derivative but are infertile amongst themselves. The species of the offspring always matches the non-Humanoid parent.
2) Mutant Animals always have offspring in sets of two. The species always matches the (non-humanoid) mother.

So far it hasn’t come up involving PCs but a couple of players have asked about it.